With their characteristic properties, aluminum electrolytic capacitors have hitherto been widely used in the energy field. Examples of the application of aluminum electrolytic capacitors include compact electronics such as cellular phones, household electric appliances such as televisions, inverter power supplies for hybrid vehicles, and storage of wind-generated electricity. In these various applications of aluminum electrolytic capacitors, there is a demand for a high-capacitance property at the voltage to be used in each application.
An aluminum electrolytic capacitor characterized by using an aluminum foil having a fine aluminum powder adhering to the surface thereof has been proposed (for example, Patent Document 1). Another example of a known electrolytic capacitor is one that has an electrode foil in which an aggregate of fine particles made of aluminum that is self-similar in the length range of 2 μm to 0.01 μm and/or aluminum having an aluminum oxide layer on the surface is adhered to one side or both sides of a smooth aluminum foil having a thickness of not less than 15 μm to less than 35 μm (Patent Document 2).
However, the methods disclosed in these documents, wherein plating and/or vacuum evaporation is used to adhere aluminum powder to an aluminum foil, are insufficient, at least for obtaining medium- to high-voltage capacitors.
Further, an electrode material for an aluminum electrolytic capacitor made from a sintered body of at least one of aluminum and aluminum alloy is disclosed as an electrode material for aluminum electrolytic capacitor (for example, Patent Document 3). This sintered body has a distinct structure, which is obtained by sintering a laminate in which aluminum or aluminum alloy powder particles are stacked while providing a space between the particles. Because of this structure, an electrostatic capacitance equivalent to or greater than that of a conventional etched foil can be obtained (paragraph [0012] of Document 3). The capacitance of this electrode material can be increased by increasing the amount or thickness of the particles of the laminate.
However, the above electrode material has a drawback in which the increase in thickness for increasing the capacitance causes difficulty in forming anodic oxide film (dielectric) on the electrode surface in the chemical conversion step. Therefore, if the capacitance per lamination unit quantity (thickness) can be increased, the thickness of the electrode material can be reduced. For example, if the capacitance per lamination thickness can be increased by 10%, the thickness of the electrode material can be reduced by 9% relative to the core material, thereby reducing the size of the capacitor.
The aluminum powder used as the raw material is obtained by classification of atomized powder (powder obtained by spattering a thin stream of molten aluminum by high-speed spray of nitrogen or the like, and then cooling the resulting particles). Among the classified powder, powder having an average particle diameter (D50) of 2 to 6 μm is used for an electrode material so as to ensure a high capacitance. It is difficult to obtain a desired capacitance when powder having a large average particle diameter is used.
However, among the powder produced by an atomizing method, powder having a small average particle diameter is only less than 50% based on the total weight. Therefore, this method has a problem of treatment of powder having a large average particle diameter. Thus, if an electrode material having a high capacitance can be obtained regardless of the average particle diameter of the aluminum powder to be used as a material, the yield of atomized powder will be greatly increased and the production cost can be reduced. As such, a need has arisen for the development of a method for producing an electrode material having a high capacitance regardless of the average particle diameter of aluminum powder.